Articulated vehicles of the track-type are generally used for travelling over peat bogs, in swampy regions, or where deep snow or the lack of road makes ordinary travel difficult or impossible. One primary use for articulated vehicles is the towing of grooming equipment for conditioning snowmobile and cross-country ski trails, and ski slopes.
Ski slopes and snowmobile trails require periodic grooming to level moguls, to scrape icy stretches and to lay an even thickness of fresh snow across slope and trail surfaces. Many of these ski resort areas extend over hundreds, even thousands of acres, part of which are precipitously steep. Similarly, snowmobile and cross-country ski trails generally span great distances over rugged and hilly terrains.
The articulated vehicles used to tow trail grooming equipment must be capable of great tractive effort so as to be able to effectively climb, descend, and traverse very steep and often irregular slopes. These articulated vehicles must be easily maneuverable, fast, stable, and controllable under the widely varying conditions usually encountered.
Various articulated all-terrain vehicles have been designed in the past. Those vehicles generally consist of a front body having two endless crawler tracks and comprising the operator's cab, and a rear body also having two endless crawler tracks and normally comprising the engine compartment. The front body is generally connected to the rear body by an articulated joint having a vertical steer axis. A longitudinal roll axis is also usually provided on one of the rear or the front body. The longitudinal roll axis on the rear body for example, is generally relative to the front body, for the forward part of the articulated joint is normally rigidly attached to the front body.
A common defect of those articulated vehicle is that when the front body tilts, from one track climbing over a mound for example, the steer axis also leans sideways. The plane of steering is thereby inclined relative to the ground or snow surface. Thus when the vehicle steers in this position, torsional and bending stresses are imposed on both body frames from the prying of the rear body on the longitudinal roll axis. Such a contorted condition causes an uneven distribution of the machine's weight over each of the four tracks.
Therefore, with conventional four-track vehicles of the like, both control over steering and hill climbing ability are usually adversely affected by track slippage and track digging-in.
As a first example of an articulated vehicle of the prior art, the Canadian Patent No. 1,077,399, issued on May 13, 1980 to Nell W. Plerson, discloses an all-terrain vehicle of the articulated type including a first unit, and a second unit pivotally and rotatably inter-connected with the first unit. The forward portion of the articulated joint is rigidly attached to the frame of the first unit. Hence, the steer axis of the joint follows the tilting movement of the front unit.
As another example of prior art articulated vehicle, U.S. Pat. No. 3,937,289, issued on Feb. 10, 1976, to Lawrence Chapman Dickinson, discloses an articulated vehicle having an articulated joint joining a fore and aft body frame members, and permitting relative rolling motion and relative yawing motion therebetween. The front portion of the joint on this vehicle is rigidly attached to the fore body member.
In another example, U.S. Pat. No. 4,966,242, issued on Oct. 30, 1990 to Bernard Baillargeon, discloses an all-terrain vehicle having an articulated joint with a vertical steer axis, and wherein the front frame is capable of oscillation about a roll axis located above the two frames and extending parallel to the longitudinal axis of the both frames. In this case, the steer axis of this vehicle is fixed to the rear frame.
In a further example, U.S. Pat. No. 5,113,958, issued on May 19, 1992, to Thomas R. Holden, discloses a vehicle where the chassis of the vehicle is pivoted about a vertical axis for limited side-to-side relative movement for steering, and pivoted about a horizontal axis for limited movement to accommodate ground irregularities. A vertical pivot pin is supported between parallel spaced apart transverse horizontal plates at the rearward end of forward chassis. A pair of tie rods with spherical bearings join the lower transverse plate to the rear chassis, permitting relative rotation about both vertical and horizontal axes. The steer axis on this vehicle is also in a fixed relationship with the front chassis.
The foregoing examples are just a few representative examples of all-terrain articulated vehicles of the prior art. It is a general condition of those vehicles that the oscillating motion of the front or of the rear body member about the longitudinal axis of the vehicle is theoretically possible only when both portions of the vehicle are in-line with one-another.
When the vehicle steers, the body portion carrying the fixed part of the articulated joint must remain substantially parallel to the actual plane of steering of the vehicle. Otherwise, the stresses applied on the frames from the misalignment of the plane of movement of the roll axis with the direction of displacement of the rear body during steering, causes an uneven pressure distribution of the tracks on the ground or snow surface. The maximum tractive effort of one, two or sometimes three tracks is thereby reduced.
Furthermore, such uneven pressure distribution of the tracks during steering causes track slippage and track digging-in. Consequently the vehicle leaves deep track traces on slopes and trails. Those track traces are ordinarily undesirable, especially when the vehicle is travelling without grooming equipment.